- Rewrite _split_index_url() to handle multiple --index-url /
--extra-index-url options on a single requirements.txt line using
regex-based parsing instead of single split.
- Cache installed_packages snapshot in collect_requirements() to avoid
repeated subprocess calls during downgrade blacklist checks.
- Add unit tests for multi-URL lines and bare --index-url edge case.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>